The present invention relates to a directional coupler and a transmitting/receiving apparatus.
In wireless transmitting/receiving systems in which transmission and reception are performed simultaneously, there is a problem in that the transmission signal can cause transmitter leakage in the receiver, degrading the reception sensitivity. In order to isolate the transmission and reception, a circulator is generally used. However, it is difficult to integrate a circulator into communication-use ICs. Moreover, the scale of the circuit is large and costs become higher.
One known method for solving this problem is to use a directional coupler. One known configuration of a directional coupler provided in a wireless transmitting/receiving system includes a first terminal connected to an antenna, a second terminal connected to a receiver, a third terminal connected to a transmitter, a fourth terminal connected to a termination impedance, a first phase shifter which is connected between the first terminal and the second terminal and causes a phase shift of π/2, a high-impedance first passive element made up of a capacitor and resistor or the like connected between the second terminal and the third terminal, a second phase shifter which is connected between the third terminal and the fourth terminal and causes a phase shift of π/2, and a high-impedance second passive element made up of a capacitor and resistor or the like connected between the first terminal and the fourth terminal (for example, refer to Yoshihiro Konishi et al., “Microwave electronic circuit technology vol. 6”, Nikkan Kogyo Shimbun, 2002, p. 55).
In a directional coupler of such a configuration, the transmission signal from the transmitter which causes transmitter leakage in the receiver is removed at the second terminal through addition to a signal of reverse phase. Hence, noise in signal received by the receiver is reduced.
However, the transmission signal is passed through the high-impedance first passive element and the second passive element. Hence, losses occur in the transmission signal outputted to the antenna. To compensate for the losses, a gain of the power amplifier provided between the third terminal and the transmitter is increased. The increased gain causes the problem of an increase in power consumption.